1. Field of the Invention
This invention relates to the destruction of radioactive graphite and silicon carbide in graphite fuel elements, and is particularly directed to a method to destroy the graphite and silicon carbide in radioactive graphite fuel elements containing small spheres of uranium oxide coated with silicon carbide, in a graphite matrix.
2. Description of the Prior Art
Radioactive graphite fuel elements generally contain small spheres of uranium oxide coated with silicon carbide in a graphite matrix. More specifically, the micro spheres consist of an inner core of fissile material or fertile material coated with a layer of graphite, then coated with a layer of silicon carbide to retain the fission products, and finally coated with an outer graphite layer. The fissile material is typically a mixture of uranium oxide, UO.sub.2, and uranium carbide, UC.sub.2. The fertile material is thorium oxide, ThO.sub.2, which is converted to fissile uranium-233 on irradiation with neutrons in a nuclear reactor. A mixture of coated fissile and fertile particles are bonded together into rods with a binder made of graphite powder and pitch. The rods are fired to carbonize the pitch.
In order to reduce the volume of radioactive waste for disposal and to render the radioactive elements into a safe form for long-term storage, a method is needed to destroy effectively the graphite matrix and the silicon carbide coating, while still retaining the radioactive material for subsequent processing. Common incineration processes require very high flame temperatures, near 1200.degree. C., to oxidize graphite, and do not oxidize or destroy silicon carbide. Thus an incinerator must treat this waste in a two-stage process, by first burning off the outer layer graphite and the graphite matrix, and recovering the microspheres in the fly ash and bottom ash. The recovered spheres are then crushed mechanically to break open the SiC and expose the inner layer of graphite. The crushed spheres are recycled into the incinerator to burn the inner layer of graphite. This exposes the radioactive material, fissile, fertile, and fission products, which must then be recovered from the bottom ash and the fly ash.
U.S. Pat. No. 3,567,412, "Gasification of Carbonaceous Fuels", discloses that the presence of sodium sulfate in an alkali metal carbonate melt is effective in promoting the oxidation of carbonaceous materials.
The destruction of the radioactive graphite in radioactive graphite fuel elements is disclosed in U.S. application Ser. No. 108,277 of R. L. Gay and D. Stelman, filed Aug. 19, 1993, titled "Method For Disposing of Radioactive Graphite in Graphite Fuel Elements", and assigned to the same assignee as the present application.
Radioactive graphite fuel elements are approximately 40 cm across the hexagonal diagonal and about 79 cm long. Criticality limits due to fuel enrichment limit the diameter of process vessels to about 20-30 cm ID. This makes processing of whole fuel elements very difficult. Mechanical size reduction of the fuel elements can be done by crushing and grinding the graphite to small particle size, about 1 cm diameter. However, this size reduction process produces many particulate fines, which require stringent containment hardware and procedures. A method is needed for treating these fuel elements without size reduction of the waste.